The objective of this proposal is to identify the mechanisms which differ between C. albicans yeast and hyphae, for control of proliferation, and for the response to nutritional signaling through the target of rapamycin pathway. In previous work we identified a regulator of hypha-to-yeast morphogenesis, Pes1. Whereas homologs of this gene are essential in all other eukaryotes studied, C. albicans hyphae continue to grow robustly during its depletion, while yeast arrest growth. Our preliminary data show that PES1-depleted hyphae respond to inhibition of Tor nutritional signaling similarly to the wild type, whil yeast fail to arrest growth appropriately. Pes1 is therefore required both for normal yeast proliferation, and for the normal response of yeast to Tor inhibition. We found Pes1 to be required for normal levels of a marker of translational activity which we adapted for use in C. albicans. We also found it to be required for progression of the cell cycle from G1 to S phase. As Pes1 localization differs between cell types and between active and inhibited Tor signaling states, we reason that some of its protein interaction partners differ between these distinct conditions, and preliminary affinity purification experiments support this idea. We aim to identify Pes1 interacting proteins by affinity purification from the distinct cell types and nutritional signaling states. To test the biological relevance of Pes1 interaction partners specific to these conditions, co- immunoprecipitation experiments with reciprocal epitope and affinity tags will be performed, and co- localization of these proteins with Pes1 in the cognate conditions will be confirmed. The role of these Pes1 interaction partners for cell-type specific proliferation control will be investigated by reverse genetic approaches. Localization of Pes1 in yeast and hyphae, and in Tor-active versus -inhibited cells will be determined using co-localization with known markers of specific organelles. Genetic analysis of central nodes of the Tor signaling pathway will answer the question whether cell-type specific signaling is intrinsic to all branches of the pathway or is specific to one branch, and will determine the epistatic and functional relationship of PES1 to these critical elements of Tor nutritional signaling. The results of this work will promote our understanding of specific proliferation control of the ubiquitous C. albicans cell type, the yeast.